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Healthy urban trees – what is required?

The urban environment can be very challenging for a tree, particularly as conditions are commonly less than optimal. Many urban trees therefore very rarely have a long life expectancy – potentially only between 7-30 years, depending on the level of landscape adversity (Foster & Blaine, 1978; Moll, 1989; Nowak et al., 2004; Roman & Scatena, 2011). In order to improve conditions and ensure that an urban tree has the potential to fulfil a full-term and healthy life therefore, many factors must be considered (Kopinga & van den Burg, 1995).

1. Space

The size of the rooting environment is directly attributable to root growth; once tree roots have reached a point where they can no longer grow in a given direction, the progressively increasing demand for water and nutrients to aid in providing the energy for growth cannot be met – because root growth is confined to a closed soil environment with a limited ‘carrying capacity’ (Day & Bassuk, 1994; Grabosky et al., 2001; Kopinga & van den Burg, 1995). Very common in urban environments, particularly where a tree is bordered by built structures on two or more sides (such as roads or footpaths); at times, the available soil is less than the drip line of the tree, even soon after planting (Jim, 2001). Unless the roots can break free of the area that the root zone is confined to and can find a source of water and nutrients elsewhere, mass will be reduced and / or growth will halt (Day et al., 2010; Sanders et al., 2013). On average, trees require 2.46m of verge between a road and path to be able to grow without (1) causing problems and (2) experiencing (major) problems (Shigo, 1991), and where trees are confined to tree pits the design of the pit will be the limiting factor for tree growth (Roberts et al., 2006).

It is also necessary to note that trees need space above ground. Where aerial space is confined, trees may need to be pruned in order to ensure they are in-keeping with their surrounding environment. Pruning, most probably on a cyclical basis so that the tree retains an artificially maximum size, introduces wounds to the tree’s structure. Such wounds are undesirable – decay may be introduced, energy is required for compartmentalisation (particularly if decay becomes serious), and photosynthetic mass is reduced (at least, temporarily) (Shigo, 1986; Shigo, 1991). The introduction and progression of decay, in particular, is likely to reduce the life expectancy of an individual.

These raywood ash have a good amount of aerial space to grow within, whilst not impeding upon the highway nearby.

Therefore, in order to ensure an individual is not adversely impacted by space, many questions must be asked prior to planting: (1) what is the ultimate size of the individual, and will the site be able to cater for its maximum size?; (2) can space for trees be designed into new areas set for construction?, and; (3) is the cost of on-going maintenance, where maximum size is not sustainable, in itself a sustainable measure, as a last resort?

Where the maximum size of an individual will exceed available space, the correct tree must thus be selected. Planting a large willow in a cramped car parking zone is likely to lead to its premature death (or removal) unless it is retained as a pollard, though planting a rowan may see the individual have a full and healthy life without any significant maintenance. Fastigiate varieties of trees (hornbeam, liquidambar, ginkgo, and oak, to name four) also exist thanks to cultivation techniques, and such individuals are particularly good for where aerial space is lacking (Patch, 1981; Ware, 1989). Such trees, incidentally, are also more likely to persist through severe wind storms with little damage (Cutler, 1991).

Unlike the raywood ash shown above, this horse chestnut lacks the aerial space needed for it to adopt a full crown (which has resulted in it being crown reduced). It also lacks the rooting space needed.

It is perhaps easier for Local Authorities to implement, particularly for the rooting environment, design requirements for new developments in the form of ample rooting space (tree pits, for example) – the Trees & Design Action Group (2014) details many measures and considerations in their recent publication Trees in Hard Landscapes. Similarly, planting schemes can be altered by Local Authorities during the planning process – such a time would be beneficial for ensuring that the right individuals are planted in the right places, with regards to their required space. Tree and Landscape Officers must therefore be heavily involved wherever vegetation is concerned. This does not however solve the issue of existing developments lacking the capacity to accomodate large trees.

2. Nutrients

The necessary abundance of nutrients within the soil is absolutely critical to the long-term survival of an urban tree (Kopinga & van den Burg, 1995; Pirone et al., 1988), though many urban soils are found to be lacking (de Kimpe & Morel, 2000). Where nutrients are therefore lacking, particularly nitrogen and potassium, the health of the individual will begin to decline – phosphorus deficiency may also be a significantly-limiting factor, where soil pH is too extreme as a result of pollution (Craul, 1994; Jim, 1998). At times, however, nutrients may not necessarily (but may indeed still) be lacking, though soil bulk density may simply be too high (Kopinga, 1991) – this can simulate the same effects as nutrient deficiencies can, given the tree simply cannot access the available nutrients.

In order to ensure trees do therefore receive the necessary amounts of nutrients required for long-term and healthy growth, many practical solutions exist. Firstly, soil can be amended via the application of either quick- or slow-release fertilisers that restore, though only on a temporary basis, soil nutrients (Davis, 2015; Pirone et al., 1988; Shigo, 1991) – in certain instances, it may be necessary to analyse the soil to ascertain the extent of any deficiency (Pirone et al., 1988), and this in itself may be somewhat costly if routinely undertaken. Where fertiliser application is perhaps not warranted, either due to cost or other limiting factor, mulch may be a preferable alternative (Davis, 2015; Sæbø & Ferrini, 2006). Scope also exists to artificially inoculate the soil with mycorrhizae so to improve nutrient uptake by trees (Harris et al., 2004) – a practice which is particularly effective when coupled with fertiliser application (Adesmoye et al., 2009; Appleton et al., 2003) – or look to improve soil conditions (improve aeration, for instance) to permit the natural succession of mycorrhizae into the soil (Saif, 1981). The amelioration of soil may also improve nutrient availability directly, particularly if the soil is compacted (Kopinga & van den Burg, 1995). As a more long-term measure however, careful species selection may be cost-effective. The planting of alder, which is a nitrogen-fixing species, can improve nitrogen availability, though utilising successional species in general will normally aid, to a degree, with soil amelioration, as well as mycorrhizal establishment, and thus nutrient availability (Põlme, 2014; Shigo, 1991; Temperton et al., 2003; Wiemken & Boller, 2002) – either for the successional species to themselves thrive via creating their own niche, or for the trees to act as nurses to other trees to be planted afterwards (or that may already exist) (Prévosto & Balandier, 2007).

Caution should however be exercised where any amendments are made to the soil. For example, whilst the use of fertiliser can certainly improve tree performance where nutrients are lacking, a dosage that is too high, or is applied too late in the growing season, is likely to be counter-productive to tree health (Benzian et al., 1974; Pirone et al., 1988; Shigo, 1991). This is a particular risk where an urban tree may already be stressed – increasing soil nitrogen causes the tree to absorb (via active transport) the nitrogen, which requires energy in itself, to then synthesise amino acids and other hormones, and to subsequently increase growth rate (Shigo, 1991). If low energy reserves exist within the tree prior to application, such application may worsen the capacity for the tree to operate in the long-term by further reducing energy levels, unless the energy ‘debt’ accumulated by the new growth is ‘paid back’ with a surplus.

3. Water

Trees require (though not in significant excess) water for a vast number of life processes, ranging from the uptake of nutrients from the soil (via active transport within root hairs) to transpirational cooling from the leaves. In fact, water demand (and subsequent stress) may be more significant in urban areas due to the urban heat island effect, which not only raises temperatures but also may reduce humidity (that in turns hastens transpiration by creating a larger ambient water vapour deficit around the leaves) (Cregg & Dix, 2001; Whitlow et al., 1992), the swathes of impervious surfaces that limit water percolation through the soil (Iverson & Cook, 2000, Roberts et al., 2006; Whitlow et al., 1992), the fact that large abundances of storm-water are transported within sealed pipe networks, and the lack of rooting space (Grabosky et al., 2001; Kjelgren et al., 2000).

A sycamore within a heavily-paved area, which will limit water availability to its rooting system. Additionally, it is impossible for this sycamore to achieve a mature size – not only is it lacking rooting space, but the flats just out of view to the right restrict its aerial space as well. This entire estate is filled with sycamore, unfortunately. Completely the wrong species choice.

Given most trees found within urban areas are transplanted, it is first critical to reduce the water stress that manifests itself in the years following planting (Pirone et al., 1988; Harris et al., 2004; Mincey & Vogt, 2014). On average, staggered watering over one season for a young tree will need to amount to between 240-640L (Buhler et al., 2006) at 15-30L intervals (Davis, 2015). Even at such an early stage in the tree’s life however, may watering be an issue – particularly where the tree is maintained by the Local Authority. Budget constraints have lead to a stalling in allocation for planting and aftercare, and compiled with the budgetary shift towards maintaining existing trees (Johnston, 2010), watering may not be feasible even at such an early stage. Potential means around such an issue may come in the form of encouraging resident involvement, which can and does work where implemented properly (Mincey & Vogt, 2014), or selecting species that are more tolerant of dry conditions (May et al., 2013; Roloff et al., 2009) – Castanea sativa, Gleditsia triacanthos, and Koelreuteria paniculata are just three examples, though particular cultivars of certain species may also display heightened tolerance to dry conditions (Percival et al., 2006). Where trees are to be planted into newly-developed areas however, there is scope for Local Authorities to require for tree pits to be designed at the planning stage that integrate storm-water management systems for irrigation purposes (Bartens et al., 2009; Coutts et al., 2012) – implementation of such a system will aid with long-term health of the trees within the area benefiting from the irrigation, particularly where storm-water can be retained for longer periods of time.

Where storm-water management systems do not exist, and their implementation is not cost-effective or practicable, management options for larger trees are largely preventative in place of reactive. Reactive measures in the form of artificially watering mature trees during drought periods are likely to be incredibly costly and probably wasteful, unless many controls are put in place to reduce wastage (May et al., 2013), though irrigation can be very successful at reducing both stress and higher than average rates of mortality (Hickman, 1993). Wherever possible therefore, the surface surrounding the trees should remain free of impervious materials, so to improve rainwater percolation, though selective removal (thinning) of denser stands within urban areas may also potentially decrease competition for soil moisture; assuming the gain in leaf area of the remaining trees does not exceed (potential) total leaf area of the pre-thinned stand. Interestingly however, mature trees are usually less likely to suffer from drought stress than young trees – tolerance increases with age, by-and-large (Niinemets & Valladares, 2006). This does not of course mean that larger, older trees can be allowed to suffer as a result of neglect.

4. Light

As light is required for photosynthesis, at least a certain amount of light is required by an individual for the synthesis of sugars – of course, shade tolerance influences exactly how much light is ‘optimal’ (Niinemets & Valladares, 2006; Valladares & Niinemets, 2008). Ensuring a tree does not suffer as a result of heavy shading is, theoretically, rather straightforward – particularly where new plantings schemes are being designed. Planting schemes, typical with new developments, can ensure that trees are planted where light conditions will be suitable. For instance, there is little justification in planting an oak within a courtyard that, even during summer, will receive very little natural light. Growth will likely be stunted (Dover, 2015; Jutras et al., 2010; Kjelgren & Clark, 1992), and such poor vigour may have adverse impacts upon the individual’s long-term health – notably when wounding occurs, and the individual cannot synthesise anti-microbial compounds in the necessary abundance (Lewinsohn et al., 1993). The planting of a beech, on the other hand, may be more justified, given its greater shade tolerance (Valladares et al., 2002; Welander & Ottosson, 1998). Similarly, where an existing group of established trees already exist within the urban environment, the planting of a young individual within the shade of the mature trees will likely be detrimental and may have longer-term implications with regards to its health. The same can also be said for narrow streets lines with tall buildings. Trees that are not shade tolerant should not be planted along such streets, as they will suffer as a result of reduced photosynthetic ability (Dover, 2015).

A small Japanese cherry resides between a large Italian alder and European lime. What chance does this tree have of achieving good form and maintaining good vigour into maturity? Probably little. It simply cannot compete.

It is also important to recognise that trees may also suffer from high light-stress. In urban areas in particular, the array of artificial surfaces are usually more reflective of light than their natural counterparts (Dover, 2015). Therefore, where an individual is situated within an area with a high percentage of solar irradiation being reflected, there is the potential risk of both photodamage and photoinhibition – this is particularly a risk where the individual may operate optimally at lower light intensities, or where nutrient deficiencies exist (Cakmak & Marschner, 1992; Critchley, 1981; Marschner & Cakmak, 1989) – which is in itself more common within urban environments (Kopinga & van den Burg, 1995). Research into the impact of very high light intensities in urban trees is however lacking, and most research has focussed on much smaller higher-tier plants. Regardless, where trees are planted, care must be taken to ensure that light levels are neither too low or too high.

5. Genetics

Because of the poor conditions of many urban environments, the genetic provenance of individuals sourced for urban tree planting schemes is incredibly important. Without optimal genetic properties that enable the individual to cope with the harsh urban setting, the individual is unlikely to achieve a significant age. Used routinely within forestry to ensure stock quality is high (Lines, 1987), there is no question that careful genetic selection of characteristics exhibited by trees that are desirable for urban areas should take place – for instance, high salt tolerance is a necessary trait for any urban tree alongside a main route to be successful.

Therefore, nurseries should be locating seed sources where the parent exhibits preferable traits (personal communication with Barcham indicates such a practice is already occurring) – as should customers be demanding such a practice. Building on the above point to provide for an example, where salt tolerance is required then sourcing seed from successful specimens in coastal areas is a distinct possibility; as is sourcing seed for sites contaminated with heavy metals from similar and existing sites (Wilkins, 1997). Despite this, evidence points towards sourcing from local trees wherever possible (Mortlock, 2000; Wilkins, 1997; Wilkinson, 2001) as, in doing so, the sourced seed is more genetically optimised for the environment in which it will grow. Long-term, this could mean the difference between premature death and survival.

6. Safeguarding

Even in spite of ensuring all aforementioned factors are in order, an individual will likely fail to achieve a full life if it suffers from significant damage in the form of vandalism or otherwise – damage via such means can introduce decay and dysfunction (Foster & Blaine, 1978; Gilbertson & Bradshaw, 1985; Kopinga & van den Burg, 1995). Vandalism is particularly a problem on young trees, and at times premature death can reach rates of 30% (Gilbertson & Bradshaw, 1985; Gilbertson & Bradshaw, 1990; Pauleit et al., 2002), though vandalism may occur on older trees as well and impacts can be very severe. Risk of damage as a result of mower damage is a further possibility, and whilst many injuries are minor, numerous injuries over time may facilitate greater dysfunction.

In order to prevent such damage therefore, it is necessary for trees to be protected from being damaged by such means. Of course, it is not practical for every single tree to be physically protected by a cage – this should only be reserved for young trees, or incredibly important trees (Harris et al., 2004). Additionally, one can never entirely remove vandalism as a risk to trees – education can however seek to reduce damage, by informing people of the benefits of trees and how to properly care for them.

However, where damage is as a result of other avoidable damage, such as mower damage, an element of control exists to safeguard trees from harm. Shigo (1986) boldly suggests that, where mower operatives are found to have damaged a tree, they should lose a day’s pay. Whilst this is perhaps not sustainable as a matter of routine, employers (such as Local Authorities and large landscaping companies) can mandate the need for acute awareness when operatives are working within the vicinity of trees. The monitoring of an operative’s work can be a further manner in which to minimise risk, as can a disciplinary if an operative continues to cause damages to trees.